It is known that vermiculite ore, a type of layer mineral (and other layer-silicate minerals containing vermiculite layers, e.g., hydrobiotite or chlorite vermiculite) can be delaminated by first exfoliating the mineral by application of heat to expand the vermiculite particle structure, or by chemical means such as treatment with hydrogen peroxide or with aqueous solutions of salts followed by washing and soaking in water to swell the vermiculite particulate structure. Such treatment is then followed by mechanically shearing the resulting expanded or swollen granules of vermiculite in water to form an aqueous dispersion of tiny particles or platelets of vermiculite known as vermiculite lamellae. Such processes for the delamination of vermiculite ore are described, for example, in U.S. Pat. Nos. 4,608,303; 4,472,478; 4,271,228; 3,791,969; 3,434,917; 3,325,340 and GB Nos. 2,007,153; 1,585,104; 1,119,305; and 1,076,786.
It is also known that aqueous dispersions of vermiculite lamellae obtained by the delamination of vermiculite ore as described above can be used to fabricate shaped articles or products such as paper, sheets or composites with other fibrous material by casting the dispersion against surfaces of various shapes and sizes and removing water from the vermiculite particles. Such dispersions can also be used to form rigid foams by gasifying the dispersion prior to removal of water from it. Examples of such product fabrication are also provided in the aforementioned references.
Recently it has been reported that vermiculite ore can be mechanically delaminated merely by subjecting an aqueous slurry of the ore to a shearing action, such as, for example, shearing by a rotary paddle, cowles blade, colloid mill, roller mill or some similar shearing macerator, until a dispersion containing vermiculite lamellae having dimensions less than 200 microns is produced. Such a process is described in U.S. Pat. No. 4,801,403.
Articles composed wholly of vermiculite ore particles or composites comprising vermiculite ore particles possess a degree of structural integrity which may be due to the great extent of overlapping of the individual vermiculite platelets or lamellae to form layers, and the strong adhesive forces generated by the high surface charge density characteristic of the vermiculite layers. Further, due to their high heat resistance, such articles comprising vermiculite are particularly suited for use, for example, as electrical and thermal insulation materials, fireproof packaging materials and refractory-facing materials.
Despite such usefulness, however, the employment of such articles has been limited in applications where there is contact with aqueous media, as such vermiculite articles tend to absorb and hold water which not only detracts from their electrical insulating qualities, but tends to weaken them physically therein imperiling structural integrity and as such may actually disintegrate during prolonged exposure.
Various methods have been proposed to impart resistance to water dispersibility in vermiculite products wherein structural integrity of the vermiculite lamellae framework is maintained in aqueous media over long periods of time. Such methods typically propose the addition of "wet-strength" improvers to dispersions of vermiculite lamellae or the incorporation of such wet-strength improvers into articles formed therefrom. These methods include, for example, post-formation treatment of vermiculite articles with aqueous solutions of electrolytes, U.K. Specification No. 1,016,385; B.P. No. 1,593,382; U.S. Pat. No. 4,608,303; and the post-formation treatment of such articles with ammonia vapor or the vapor of an amine, U.S. Pat. No. 4,219,609. Other methods include treatment of dispersions of vermiculite lamellae used to form products with solutions of inorganic ions, U.S. Pat. Nos. 4,269,628 and 4,497,869; the incorporation of a source of ammonia or ammonium ions, preferably urea, into a dispersion of vermiculite lamellae, U.S. Pat. No. 4,539,046; and the incorporation of a urea-formaldehyde or melamine-formaldehyde resin into a dispersion of vermiculite lamellae, U.S. Pat. No. 4,485,203.
Such methods, however, merely help to maintain the structural integrity of articles composed of vermiculite in aqueous media, but do not decrease or halt surface wetting of such articles. Such surface wetting leads to penetration and absorption of water therein, which limits the use of such articles as electrical insulators in applications where contact with aqueous media is a possibility. Indeed, several of the aforementioned methods actually increase water absorption by rendering the surface of vermiculite containing articles hydrophilic in nature thus increasing surface wetting. Moreover, those methods mentioned above which employ ionexchange processes as a wet-strength improver may be disadvantageous from the standpoint that such improvement in wet-strength may decrease or become non-existent when vermiculite articles treated with such ions are exposed to media containing exchangeable competing ions. Further, those processes mentioned above which employ post-article formation treatment methods are additionally non-desirable from the standpoint of the need to avoid extra processing steps, followed by thorough rewetting and washing to remove excess salts, ammonia and the like, therein followed by thorough redrying. Those disadvantages are further compounded in the manufacture of vermiculite foams due to the thickness and absorbent nature of the foam.
Accordingly, it is an object of this invention to provide a process for the production of shaped vermiculite articles having improved resistance to surface wetting in aqueous media with correspondingly decreased water penetration and absorption. It is still a further object of this invention to provide a method for producing said formed vermiculite articles from dispersions of vermiculite lamellae.